Lithographic duplicators capable of duplexing, that is printing on both faces of a copy sheet in a single pass, are conventional equipment. These duplicators are normally arranged with tandem print heads between which is disposed a sheet turnover device which may be rendered operative or disabled depending upon whether duplex printing or one-side, two-color printing is desired. Such machines are also capable of operating in a simplexing mode to provide single-face, single impression printing, and in this case the printing is done on one of the print heads while the other print head idles to pass the copy sheet through, the turnover device normally being disabled during this procedure.
In the simplexing mode as just described, the duplicator is essentially operated exactly like a single-head duplicator and requires a cleanup and preparation operation on the printing head in use, each time a printing job is completed and before another can be started. It will be readily appreciated that in equipment of this type the copy sheet must pass through the nip of the printing cylinders on the non-printing head just as if it were being printed, except for the fact that there is no image on the blanket cylinder. For this reason it is the second head of the two which is normally used for printing to avoid offset and tracking problems on the non-printing head.
Recently there has been developed high-speed automated dual-head duplicating equipment, especially applicable in short run systems use, which is arranged to take prepared masters from a single supply source and feed them to the two print heads and automatically install them thereon, and this has very materially speeded up the duplexing operation of such machines and simplified the operator's task. Such a machine is shown in U.S. Pat. No. 3,987,722 to E. L. Goodwin, assigned to the assignee of the present application.
In none of the foregoing prior art arrangements was there provided any way to augment the production of such dual-head duplicators when operating in a simplexing mode.